1. Field of the Invention
The present invention pertains to the field of Power Inverter Circuits used to convert direct current, D.C., to alternating current, A.C. In general, such apparatus have been designed to receive a D.C. input, which in turn is converted to an A.C. source suitable for driving an A.C. receptive load. The disclosed apparatus controls the power inverter circuit by altering the magnetic characteristics of the inductor connected in parallel with the primary of the transformer, which supplies current or voltage to the control pin of the power inverter transistors. The alteration of the magnetic characteristics of the inductor may be influenced by a voltage derived from a current transformer in series with the A.C. receptive load.
2. Prior Art
A common power inverter application is to provide compatible A.C. power to operate different loads such as fluorescent lamps, Cold Cathode Fluorescent lamps, and electro-luminescent panels, halogen lamps, H.I.D. lamps, Metal Halide lamps. This power inverter may be used as a switching power supply for numerous other types of loads. Fluorescent lamps are commonly used to provide illumination, particularly in industrial environments where their economy of power utilization is highly desirable. Because of their greater efficiency in converting electricity to light, the cost of utilization is significantly reduced when compared to incandescent lighting.
Cold Cathode Fluorescent lamps (CCFLs) are used to backlight Liquid Crystal Displays (LCDs) in computer applications while Electro-luminescent (EL) panels are used to backlight LCDs, key switches, and other devices in many applications. Their popularity is due to high efficiency and small size. These devices require a high voltage ac current to drive them. Power inverter circuits commonly supply this power.
A common limitation of these Power Inverter Circuits has been that they have required sophisticated circuitry to vary the brightness of the above-mentioned lamps and other loads. Most modern fluorescent lamp ballasts utilize a D.C. to A.C. inverter circuit to strike and supply operating power to the lamps. Many power inverter circuits commonly supply a non-variable voltage to the load. As control circuitry is added to accomplish regulation or dimming of the light source, the complexity and cost has historically increased dramatically while the reliability and manufacturing consistency have decreased. Additionally, the control circuitry often interacts in an undesirable manner with various aspects of the circuitry thereby requiring further complexity to compensate for these effects. Furthermore, dimming at low levels and from multiple sensory control elements is limited.
Likewise, switching power supplies and high frequency supplies for driving halogen lamps commonly suffer from the same limitations.
In Biegel U.S. Pat. No. 7,274,574, auto-dimming of a Magnetically Controlled Power Supply is taught. There is a level where the inductance of the primary becomes low enough that the circuit fails to function and the circuit turns off.
The present invention addresses the above limitations. The first is that an inductor is placed in parallel with the Control Transformer primary to control the amplitude of the output of the power inverter circuit. Secondly, the present invention teaches methods in which multiple control elements can control the output level interactively. The third is that the apparatus described herein is isolated and independent from the drive circuitry. It requires very few components and does not require complex feedback loops to control the inverter output level. Fourthly, when used to drive a fluorescent lamp load, a series resonant circuit comprised of an inductor and resonant capacitor is often used to boost the voltage level to that required to strike and operate the lamp. By adding a Current Transformer in series with the lamp, a secondary coil is used to generate a feedback voltage, this voltage can be used to supply the control current on a delayed basis and thereby provide full start up power to the fluorescent lamp load independent of the setting of any dimming or level controls. Alternatively, in conjunction with a simple RC timer, the modified power inverter circuit is also able to strike the lamps at a very low dimming level. Additionally the feedback voltage can be supplied to a variety of feedback circuits to condition the signal or feed it through a microprocessor to tailor the input to supply the desired effect. Various input devices can be used to control the output level.